Habitats, host plants, and preliminary biological notes are documented for Oncocephala promontorii Péringuey, 1898 for the first time. The beetle occurs in open areas of coastal grassland, forest margin, and disturbed urban areas where their Convolvulaceae host plants are found. Seven host plants are verified for the species in the field—Convolvulus arvensis L., Convolvulus farinosus L., Ipomoea batatas (L.) Lam., Ipomoea ficifolia Lindl., Ipomoea obscura (L.) Ker Gawl., Ipomoea wightii (Wall.) Choisy, and an unidentified Ipomoea sp. Ipomoea purpurea (L.) Roth is recorded as a host from museum specimen labels. Two additional plants, Ipomoea cairica (L.) Sweet and Ipomoea coccinea L., showed feeding traces that we attribute to O. promontorii. The larvae are solitary miners that produce curvilinear to vermiform mines in the host plant leaves. Pupation is solitary in a separate pupal chamber/mine in the leaf. Imagines are generally solitary, exposed folivores that produce narrow linear feeding trenches in the dorsal leaf surface. Copulation is commonly observed on the dorsal leaf surface.